This invention relates to printers, and, more particularly, to the substrate component of the print head of thermal ink jet printers.
Thermal ink jet print heads operate by rapidly heating a small volume of ink, causing it to vaporize and expand, and ejecting the ink through an orifice to strike a piece of paper. When a number of orifices are properly arranged, they form a dot matrix pattern. The properly sequenced operation of each orifice causes a pattern of characters or images to be printed upon the paper as the print head is moved past the paper. The thermal ink jet printer is fast but quiet, as only the ink strikes the paper, produces high quality printing, and can be made compact and portable.
In one design, the print head includes an ink reservoir and channels to supply the ink to the point of vaporization, an orifice plate in which the individual orifices are formed in the required pattern, a series of thin film heaters, one below each orifice, and a substrate which forms the back wall of the ink channel and upon which the heaters are supported. Each heater includes a thin film resistor and appropriate current leads. To print a single dot of ink, electrical current from an external power supply is passed through a selected heater. The heater is ohmically heated, in turn heating the adjacent ink and causing a droplet of ink to be ejected through the adjacent orifice to the paper.
The present invention is concerned with the construction of the substrate. The substrate supports the resistors and their current carrying leads, and must therefore be electrically nonconducting in the sense of serving as an insulator between the laterally adjacent resistors and leads. The substrate must have a low enough thermal conductivity to limit the portion of the heat pulse of the resistors conducted into the substrate. Otherwise, excessive driving energy may be required. The substrate must, however, have sufficient thermal conductivity that the portion of the heat pulse of the resistor conducted into the substrate is dissipated. Otherwise, there may be a heat buildup under the resistor that degrades print quality and reduces resistor reliability. Meeting the requirements of low electrical conductivity and moderate thermal conductivity necessitates care in the selection of the substrate construction. In addition, the substrate must have good mechanical strength to withstand the hydrodynamic forces produced during the ink ejection cycle.
Substrates have been conventionally fabricated from pieces of silicon or glass, covered with a dielectric layer such as silicon dioxide. These constructions are fully operable and acceptable. Nevertheless, there is a continuing need for improved substrate materials and designs that permit less costly fabrication and better performance of the substrate and the print head. The present invention fulfills this need, and further provides related advantages.